The present invention relates to optical fiber couplers. Devices referred to as "couplers" are commonly used in fiber optic communications systems to transfer light from one fiber to another. Optical fibers typically include a core and a cladding surrounding the core. Although light is commonly regarded as traveling within the core of a single-mode fiber, a so-called "evanescent field" spreads into the cladding.
One type of evanescent coupler, commonly referred to as an "overclad tapered" coupler, utilizes fibers which have been stretched at preselected coupling regions. The coupling regions are surrounded by a common matrix, referred to as an "overcladding". The core and cladding of each fiber taper gradually, on both sides of the coupling region, from the full diameter in unstretched regions of the fiber down to minimum diameters within the coupling region. The diameters of the claddings within the coupling regions are comparable to the diameters of the cores outside the coupling regions. Light transmitted along one of the fibers passes through the coupling region of the fiber principally by way of the narrowed cladding. As a useful first approximation, the narrowed claddings can be considered as forming the cores of further coupling-region optical waveguides, and the overcladding can be thought of as forming the claddings of each such coupling-region waveguides. Light passing along one such coupling-region waveguide will have an evanescent field extending to the coupling-region waveguide of the other fiber. Light transfers from one such waveguide to the other, and thus transfers from fiber to fiber within the coupling regions. As disclosed in commonly assigned U.S. Pat. Nos. 4,931,076 and 5,011,251, overclad tapered couplers can be manufactured by a process in which the two fibers are enveloped in a tube of the overcladding material; the tube is heated and collapsed onto the fibers; and the entire assembly is pulled to stretch the fibers and form the coupling regions.
"Fused-fiber" couplers can be made by fusing two fibers and then tapering them. Fused-fiber couplers function in a manner similar to that described above with respect to overclad couplers. When the fiber cladding diameter becomes sufficiently small, the composite of the core and cladding functions as the light guiding portion of the waveguide in the coupling region, and the surrounding air functions as the cladding. Light power propagating in the first fiber couples to the coupling-region waveguide (the narrowed cladding of the first fiber). The evanescent mode field of light traveling in the first fiber expands in the fused tapered coupling-region waveguide of the first fiber so that it couples to the fused tapered coupling-region waveguide of the second fiber.
The principles of the present invention can be employed in conjunction with both fused fiber couplers and overclad couplers.
The amount of power transferred from one fiber to another in an evanescent coupler depends upon factors including the wavelength of the light, the length along which coupling occurs, commonly called the "coupling length", and the difference between the propagation constants of the light paths in the regions where coupling occurs. The "propagation constant" is a measure of the speed of propagation of the light along a path. The propagation constant is commonly denoted by the symbol .beta., whereas the difference in propagation constants within the coupling regions of an evanescent coupler is commonly denoted by .DELTA..beta.. As discussed in greater detail below, the propagation constant of light passing through an optical waveguide depends upon the diameter of the core; the indices of refraction of the core and cladding, and the wavelength of the light.
Where the light paths of a coupler are formed from identical fibers, .DELTA..beta. is zero. In this case, complete (100%) coupling of the optical power from one light path to the other is possible. Commonly assigned U.S. Pat. No. 5,011,251 teaches an achromatic overclad tapered coupler with light paths made from different fibers. One fiber is a standard fiber having a core of high refractive index and a cladding of lower refractive index. In certain embodiments, the other fiber may be a three-layer structure including a core of high refractive index, an outer cladding of low refractive index and a thin inner cladding having a refractive index lower than that of the outer cladding disposed between the core and the outer cladding. In this structure, .DELTA..beta. is not zero. Complete optical power transfer is not possible. This phenomena is utilized in couplers made in accordance with U.S. Pat. No. 5,011,251 to provide couplers that divert a substantially constant preselected portion of the light from one optical fiber to another over a relatively wide range of wavelengths.
Other couplers are designed to provide wavelength-selective performance. For example, in a wavelength division multiplexing transmission scheme, a single fiber may carry light at several slightly different wavelengths. Each wavelength carries a separate stream of information. A wavelength selective device can be used at a point where the fiber branches to direct one wavelength onto one branch of the fiber and to direct the other wavelengths onto the other branch. Several transmitters and/or receivers belonging to different telecommunications customers can be connected to a single main fiber by wavelength-selective couplers. Each coupler is adapted to couple only a narrow band of wavelengths between the main fiber and a spur fiber leading to the particular transmitter or receiver while leaving all other wavelengths on the main fiber. Signals intended for the particular customer are sent at the wavelength associated with that customer.
A wavelength-selective coupler can be made by using two different fibers. Properties such as fiber diameter and refractive index profile are selected for each fiber so that the dispersion in the two fibers differs. "Dispersion" is the variation in .beta. with wavelength. Thus, one fiber has .beta. which varies rapidly with wavelength (high dispersion), whereas the other fiber has .beta. which varies only gradually with wavelength (low dispersion). The properties of the fibers are selected so that both fibers have the same .beta. for light at or near a desired operating wavelength. In this case, .DELTA..beta. has a large value at all wavelengths except within a narrow passband around the operating wavelength. This tends to suppress coupling at wavelengths outside of the passband, so that only light within the passband is coupled from one fiber to the other.
If cladding diameter is one of the fiber differences that is employed to achieve the desired .DELTA..beta., the different claddings will form coupling-region waveguides of different diameters when the fibers are stretched. However, this approach implies that at least one fiber will have a cladding diameter different from the standard cladding diameter commonly used for fibers in communications systems. This leads to significant practical difficulties in connecting the non-standard fiber to other fibers in the system.
U.S. Pat. No. 4,976,512 teaches a fused-fiber coupler incorporating a standard step-index fiber and a "W-index" fiber to achieve narrowband coupling between two fibers. The W-index fiber of U.S. Pat. No. 4,976,512, like the three-layer fiber of the aforementioned U.S. Pat. No. 5,011,251, includes a core of high refractive index, an outer cladding of low refractive index and an inner cladding having refractive index lower than that of the outer cladding. In the '512 patent, however, the inner cladding has an appreciable thickness and substantially influences the dispersion characteristics of the fiber, so as to provide a steeply-sloping dispersion curve. However, optical fibers having depressed index cladding regions surrounding the core, such as the depressed index region of the W-index fiber of the '512 patent, experience significant loss of optical power in the tapered regions of the fiber due to non-adiabatic mode coupling.
It is therefore an object of the invention to provide a narrow band fiber optic coupler that exhibits low excess loss.